1. Field of the Invention
This invention relates generally to audio playback circuits and more particularly to audio output drivers for reducing electromagnetic interference and improving channel isolation.
2. Description of Related Art
As is known, integrated circuits are used in a wide variety of electronic equipment, including portable, or handheld, devices. Such handheld devices include personal digital assistants (“PDA”), CD players, MP3 players, DVD players, AM/FM radio, pagers, cellular telephones, computer memory extensions (commonly referred to as a thumb drive), et cetera. Each of these handheld devices includes one or more integrated circuits to provide the functionality of the device. For example, a thumb drive may include an integrated circuit for interfacing with a computer (for example, a personal computer, a laptop, a server, a workstation, et cetera) via one of the ports of the computer (for example, Universal Serial Bus (“USB”), parallel port, et cetera) and at least one other memory integrated circuit (for example, flash memory). As such, when the thumb drive is coupled to a computer, data can be read from and written to the memory of the thumb drive. Accordingly, a user may store personalized information (for example, presentations, Internet access account information, et cetera) on their thumb drive and use any computer to access the information.
As another example, an MP3 player may include multiple integrated circuits to support the storage and playback of digitally-formatted audio (that is, formatted in accordance with the MP3 specification). One integrated circuit may be used for interfacing with a computer, another integrated circuit for generating a power supply voltage, another for processing the storage and/or playback of the digitally formatted audio data, and still another for rendering the playback of the digitally formatted audio data audible.
Integrated circuits have enabled the creation of a plethora of handheld devices; however, to be “wired” in today's electronic world, a person deploys multiple handheld devices. For example, one may own a cellular telephone for cellular telephone service, a PDA for scheduling and another for an address book, one or more thumb drives for extended memory functionality, an MP3 player for storage and/or playback of digitally recorded music, a radio, et cetera. Thus, even though a single handheld device may be relatively small, carrying multiple handheld devices on one's person can become quite burdensome.
Many of these devices also use headphones or other forms of speaker output for audio signal playback. A problem for highly-integrated circuits is that high-frequency noise from the integrated circuit carries through to the headphone or speaker leads. Filtering devices external to an integrated circuit have been used to mitigate the noise; however, these filtering devices can degrade the quality of the audio signal received at the speaker. The signal degradation can result in higher distortion, lower inter-channel isolation, and altered frequency response.
Therefore, a need exists for an audio output driver of an integrated circuit that provides improved audio channel performance and electromagnetic interference noise reduction.